Magnetic devices that use a flow of spin-polarized electrons are of interest for magnetic memory and information processing applications. Such a device generally includes at least two ferromagnetic electrodes that are separated by a non-magnetic material, such as a metal or insulator. The thicknesses of the electrodes are typically in the range of 1 nm to 50 nm. If the non-magnetic material is a metal, then this type of device is known as a giant magnetoresistance or spin-valve device. The resistance of the device depends on the relative magnetization orientation of the magnetic electrodes, such as whether they are oriented parallel or anti-parallel (i.e., the magnetizations lie on parallel lines but point in opposite directions). One electrode typically has its magnetization pinned, i.e., it has a higher coercivity than the other electrode 50 and requires larger magnetic fields or spin-polarized currents to change the orientation of its magnetization. The second layer is known as the free electrode and its magnetization direction can be changed relative to the former. Information can be stored in the orientation of this second layer. For example, “1” or “0” can be represented by anti-parallel alignment of the layers and “0” or “1” by parallel alignment. The device resistance will be different for these two states and thus the device resistance can be used to distinguish “1” from “0.” An important feature of such a device is that it is a non-volatile memory, since the device maintains the information even tens of nanometers when the power is off, like a magnetic hard drive. The magnet electrodes can be sub-micron in lateral size and the magnetization direction can still be stable with respect to thermal fluctuations.
In conventional magnetic random access memory (MRAM) designs, magnetic fields are used to switch the magnetization direction of the free electrode. These magnetic fields are produced using current carrying wires near the magnetic electrodes. The wires must be small in cross-section because memory devices consist of dense arrays of MRAM cells. As the magnetic fields from the wires generate long-range magnetic fields (magnetic fields decay only as the inverse of the distance from the center of the wire) there will be cross-talk between elements of the arrays, and one device will experience the magnetic fields from the other devices. This cross-talk will limit the density of the memory and/or cause errors in memory operations. Further, the magnetic fields generated by such wires are limited to about 0.1 Tesla at the position of the electrodes, which leads to slow device operation. Importantly, conventional memory designs also use stochastic (random) processes or fluctuating fields to initiate the switching events, which is inherently slow and unreliable (see, for example, R. H. Koch et al., Phys. Rev. Lett. 84, 5419(2000)).
In U.S. Pat. No. 5,695,864 and several other publications (e.g., J. Slonckewski, Journal of Magnetism and Magnetic Materials 159, LI (1996)), John Slonckewski described a mechanism by which a spin-polarized current can be used to directly change the magnetic orientation of a magnetic electrode. In the proposed mechanism, the spin angular momentum of the flowing electrons interacts directly with the background magnetization of a magnetic region. The moving electrons transfer a portion of their spin-angular momentum to the background magnetization and produce a torque on the magnetization in this region. This torque can alter the direction of magnetization of this region and switch its magnetization direction. Further, this interaction is local, since it only acts on regions through which the current flows. However, the proposed mechanism was purely theoretical.
Spin-transfer torque magnetic random access memory (STT-MRAM) devices hold great promise as a universal memory. STT-MRAM is non-volatile, has a small cell size, high endurance and may match the speed of static RAM (SRAM). A disadvantage of the common collinearly magnetized STT-MRAM devices is that they often have long mean switching times and broad switching time distributions. This is associated with the fact that the spin-torque is non-zero only when the layer magnetizations are misaligned. Spin transfer switching thus requires an initial misalignment of the switchable magnetic (free) layer, e.g. from a thermal fluctuation. Relying on thermal fluctuations leads to incoherent reversal with an unpredictable incubation delay, which can be several nanoseconds.
Spin-transfer torque magnetic random access memory (STT-MRAM) devices use current or voltage pulses to change the magnetic state of an element to write information. In all STT-MRAM devices known to date, voltage/current pulses of both positive and negative polarities are needed for device operation. For example, positive pulses are needed to write a “1” and negative polarity pulses are needed to write a “0”. (Of course, the definition of which magnetic state represents a “1” and which a “0” is arbitrary.) This magnetic element typically has two possible states, magnetization oriented either “left” or “right”, parallel or antiparallel to the magnetization of a reference layer in the device. These two magnetic states have different resistances, which can be used to read-out the information electrically.
Using present complementary metal-oxide-semiconductor (CMOS) technology, circuitry is needed to control the signals to STT-MRAM cells. Prior STT-MRAM devices required bipolar sources and the bit cells were set to one state by one polarity and the other state by the other polarity, i.e. unipolar That is, the source needed to be able to provide both polarities because each polarity only could write either “0” or “1”. Although reading can be done with a unipolar voltage/current source, writing information required a bipolar source.